The long-term objective of this investigation is to employ an experimental feline model to identify the pathological mechanism responsible for aplastic anemia--an important human hematological disease with poor prognosis, obscure pathogenesis and preleukemic significance. Feline erythroid aplasia is a common, naturally occurring disease which can be induced rapidly and efficiently experimentally in cats of all ages by infection with the KT isolate of FeLV combined with concomitant single-bolus corticosteroid treatment. Specific aims are to determine whether FeLV-KT-induced erythroid aplasia in cats is initiated by viral damage either directly to marrow erythroprogenitor cells or to the erythrosustentacular cells in the marrow microenvironment and to investigate treatment of experimental FeLV-induced erythroid aplasia by infusion of antiviral serum plus autologous preinfection cryopreserved marrow cells. Investigations in FeLV-KT inoculated preanemic and anemic cats will employ techniques developed for culture of feline erythroid and granulocyte-macrophage colony-forming units, assay of marrow-adherent (macrophage-reticular) cell cultures for erythroid colony burst-promoting and -inhibiting activity, site and frequency of integration of FeLV-KT-specific DNA sequences in marrow cells after in vivo and in vitro marrow cell infection and sequential ultrastructural evaluation of marrow stromal-erythropoietic cell-sinusoid relationships.